In Python how can I name the active selection Low Poly, join the others and name them High Poly?
I hope that's what you're looking for.

Select the low-poly object and run the script to add a &lt;Name&gt;_LowPoly suffix to the objects name in the first place, join all other mesh objects of the collection and rename the result to &lt;Name&gt;_HighPoly:

import bpy

suffix = "_LowPoly"
act_obj = bpy.context.view_layer.objects.active

# Append the suffix to the active object
if suffix not in act_obj.name:
    act_obj.name = act_obj.name + suffix

# Get the collection of the active object
obj_coll = act_obj.users_collection[0]

# Create an empty list
high_poly_objects = []

# Append all mesh objects (of the collection) to the list
for ob in obj_coll.objects:   
    if not "Low" in ob.name and ob.type == 'MESH':
        high_poly_objects.append(ob)

# Deselect all objects
bpy.ops.object.select_all(action='DESELECT')

# Select all High-Poly objects to call the join operator
for ob in high_poly_objects:
    ob.select_set(True)
    bpy.context.view_layer.objects.active = ob

# Join all objects selected before
bpy.ops.object.join()

# Get the object in context and re-name it
if "High" not in bpy.context.object.name:
    bpy.context.object.name += "_HighPoly"

# Optional -&gt; Invert the selection for fun
for ob in obj_coll.objects:
    if "Low" in ob.name:
        bpy.context.view_layer.objects.active = ob
        ob.select_set(True)
    else:
        ob.select_set(False)




You can also override the context of bpy.ops.object.join() to avoid any viewport selection changes. Select all objects, make the low-poly object the 'Active Object' and run the script:

import bpy

hp_suffix = "HIPOLY"
lp_suffix = "LOWPOLY"

C = bpy.context
scene = C.scene

ob_active = C.active_object
if not ob_active.name.endswith(lp_suffix):
    ob_active.name += "_{}".format(lp_suffix)

obs = []
for ob in C.selected_editable_objects:
    if ob.type == 'MESH' and ob != ob_active:
        obs.append(ob)

if len(obs) &gt; 1:
    c = {}
    c["object"] = c["active_object"] = obs[0]
    c["selected_objects"] = c["selected_editable_objects"] = obs
    bpy.ops.object.join(c)

if not obs[0].name.endswith(hp_suffix):
    obs[0].name += "_{}".format(hp_suffix)